1. Field of the Invention
The present invention relates to a fluorescent complex and a lighting system possessing a high level of luminosity and a prolonged service life.
2. Background Art
In recent years, the luminosity and service life of LED elements have been significantly improved, and a wide range market development including illumination applications is under way.
For LED elements using inorganic fluorescent substances which are currently mainly adopted, the luminescence efficiency is being significantly improved. In particular, it is said that the luminescence efficiently of white LEDs would excel that of fluorescent lamps in the future. When LEDs are used in lighting systems, in many applications, the LEDs should be excellent in luminescence efficiency, as well as in color rendering properties. The present situation, however, is that LEDs using only inorganic fluorescent substances cannot satisfy all of these property requirements without difficulties.
The concept that organic fluorescent substances are used in LEDs is already known. At the present time, however, LEDs using organic fluorescent substances as the fluorescent substance have not been put to practical use in illumination applications due to the presence of the following problems.
1) In particular, when near ultraviolet LEDs, which are currently being mainly adopted, are used as a light source and organic fluorescent substances are used in LEDs using luminescent materials for R, G, and B, a deterioration in organic compounds by ultraviolet light is significant, because organic compounds are generally weak against ultraviolet light.
2) Since organic fluorescent substances sometimes causes a variation in a fluorescence spectrum depending upon its concentration, the regulation of the spectrum is difficult. Further, the fluorescence intensity depends upon the concentration, and, thus, concentration quenching disadvantageously takes place in a high concentration region.
3) The fluorescence spectrum sometimes disadvantageously varies depending upon the type of polymer dispersed in the organic fluorescent substance.
In general, the fluorescent substance formed of a rare earth complex has the following advantages over conventional organic fluorescent substances.
1) The luminescence wavelength is characteristic of rare earthes and thus is less likely to be influenced by the coloring matter and the type of polymer to be dispersed, and, thus, the fluorescence spectrum is stable.
2) Although the ligand is an organic compound, upon the excitation of the ligand through the absorption of light, the state is returned to the ground state by energy transfer to the central element. Accordingly, the opportunity for causing an irreversible chemical change from the excited state is reduced, and, thus, durability against ultraviolet light can be expected.
However, a further improvement in luminosity and service life is required for developing general illumination markets. Stability against a photochemical reaction of the ligand per se may be mentioned as a property which greatly affects the durability. Fluorescent substances exposed to light from LED are exposed to severe conditions such as strong heat and light and thus are likely to be deteriorated radically (oxidatively). A chemical change in ligand lowers a coordinative ability, resulting in the removal of the ligand. In some cases, this often deteriorates fluorescence intensity, and the altered ligand is causative of deactivation.
On the other hand, in order to realize high luminosity, the solubility or dispersibility of the fluorescent complex in the resin should be large. When the fluorescent substance is present as particles in the resin due to low solubility or dispersibility, light scattering occurs and makes it impossible to provide satisfactory luminosity.
The polymer in which the rare earth complex is dissolved significantly affects the luminosity of the LED element. Specifically, rare earth complexes, particularly europium complexes, when C—H bond or O—H bond is present around ions, cause quenching as a result of vibrational deactivation. That is, when C—H bond or O—H bond is present in the polymer in which the rare earth complex is dissolved, there is a tendency that the fluorescence intensity is disadvantageously attenuated. Eliminating the whole of the above bonds from the polymer, however, is practically impossible.
For example, rare earth complexes to which a crown ether has been coordinated are also known (see, U.S. Pat. No. 6,656,608). In such conventional complexes, oxygen constituting the ether group is a coordinating group. Since, however, the coordinating ability is relatively low, there is room for improvement in stability as the complex. Further, regarding complexes described in U.S. Pat. No. 6,656,608 which have a crown ether as a ligand, there is also room for improvement in the effect of shielding against C—H bond or O—H bond present near the complex when the complex is dissolved in the resin or the like.